When we use our debit cards or any card with a chip inside of it that gets immediately read when scanned, how is the memory inside the chip retrieved since the card isn't exactly "turned on" much like a standard computer?
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14You might want to clarify if you're talking about contactless specifically. Regular cards with metal contacts are powered up when inserted.– chrylis -cautiouslyoptimistic-Commented Oct 4, 2018 at 13:59
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1It works just like a regular desktop computer. When you plug it in, whatever you plug it into provides the power. When you don't plug it in, it doesn't work.– Jörg W MittagCommented Oct 5, 2018 at 6:05
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2In essence, wireless electricity. Have a look at RFID.– MonkeyZeusCommented Oct 5, 2018 at 15:13
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1It's not [read without a power source]. It's turned on exactly like a computer.– NobodyCommented Oct 6, 2018 at 12:03
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2Please don’t put answers in comments. There are contradictory answers here in the comments, and the incorrect ones can’t be fixed.– prlCommented Oct 7, 2018 at 1:17
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3 Answers
You may find the answer in Wikipedia Smart card - Contactless smart cards :
Contactless smart cards
A second card type is the contactless smart card, in which the card communicates with and is powered by the reader through RF induction technology (at data rates of 106–848 kbit/s). These cards require only proximity to an antenna to communicate. Like smart cards with contacts, contactless cards do not have an internal power source. Instead, they use an inductor to capture some of the incident radio-frequency interrogation signal, rectify it, and use it to power the card's electronics.
There is a power source... the card terminal.
When you insert your chip card with metal contacts, the terminal applies power and communicates with the card over a serial pin.
For contactless cards like RFID cards, power is transmitted from the reader, like a radio transmitter, and received by the card. This small amount of power in the air is enough to turn on the card and for the card to send a very small amount of power back. If you cut open an RFID card, you'll undoubtedly see it's antenna. Something like this:
(Image from: https://www.analogictips.com/rfid-tag-and-reader-antennas/)
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2It may be worth noting the same principles are used for "wireless" charging of toothbrushes, smart phones, etc.– DavidCommented Oct 5, 2018 at 9:37
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2you don't even have to cut open the RFID card. If you have a card with a RFID chip (or, even better, a Chip On Paper card - which is made of paper) you can put it against a bright source of light and see the inside Commented Oct 5, 2018 at 11:18
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2It may also be worth noting that, like the diagram above, the antenna often coils around the entire of the card not just a small area around the chip. If you made a 1cm cut into any part of the card's edge, it is likely it would cease to work. This gives you some idea of how it is able to absorb sufficient power. encrypted-tbn0.gstatic.com/… Commented Oct 5, 2018 at 15:41
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@SLC That image you linked wouldn't work, as there is only 1 contact with the coils, and the coils are shorted in a circle, no energy would get to the chip– FerrybigCommented Oct 5, 2018 at 19:55
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@Ferrybig: Cards use lots of really thin wires packed really close together. There would typically be four wires going out from the chip--two going around each loop. It might not matter if the wires go straight from the chip to the outer loop, or make one trip around the inner loop, so the picture might be correct if one recognizes that the loops aren't thick wires, but rather many wraps of a really thin wire.– supercatCommented Oct 5, 2018 at 22:38
The other answers correctly describe how the RFID chip is powered but not how it is read by the reader.
Transmitting a message over RF takes a relatively large amount of energy -- too much for the tiny RFID chip. So instead, it "cuts its own cord" momentarily. The reader senses this change and treats it as a digital 1 (for example). Then the RFID chip goes back to its original state and the reader senses 0.
